linux-kernel - Re: [PATCH RFC] select_idle

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Message-ID: <20160405192829.vjv7z4xzpr64kcwe@floor.thefacebook.com>
Date:	Tue, 5 Apr 2016 15:28:29 -0400
From:	Chris Mason <clm@...com>
To:	Bastien Bastien Philbert <bastienphilbert@...il.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Matt Fleming <matt@...eblueprint.co.uk>,
	Mike Galbraith <mgalbraith@...e.de>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC] select_idle_sibling experiments

On Tue, Apr 05, 2016 at 02:43:09PM -0400, Bastien Bastien Philbert wrote:
> On Tue, Apr 5, 2016 at 2:08 PM, Chris Mason <clm@...com> wrote:

[ ... ]

> >
> > I tried a few variations on select_idle_sibling() that preserved the
> > underlying goal of returning idle cores before idle SMT threads.  They
> > were all horrible in different ways, and none of them were fast.
> >
> > The patch below just makes select_idle_sibling pick the first idle
> > thread it can find.  When I ran it through production workloads here, it
> > was faster than the patch we've been carrying around for the last few
> > years.

[ ... ]

> >
> Here is my concern, do you test this on standard scheduler workloads
> or was this just written for Facebook's internal workloads. I am going
> to test this later because frankly this may cause a regression on my
> system which has only 4  cores so a idle CPU is probably less common
> for a small amount of time. I am wondering however if Ingo has any
> complains before I test this to see if it causes a regression or a bug
> on my system.  Ingo do you have any thoughts on this or would you like
> me to just test this?  Bastien

Pretty much every commit to select_idle_sibling over the last few years
was somehow trying to preserve or improve the select-idle-cores-first
functionality I just ripped out.  So, it's safe to assume it'll break
something ;)

-chris